In general, intermetallic parts may be formed by powder metallurgy and exhibit improved structural or performance characteristics over parts formed of other materials or compounds. Iron aluminides based on Fe.sub.3 Al and FeAl, for instance, are suitable for use in a variety of structural applications and for a variety of components. The combination of low density, excellent oxidation and sulfidation resistance, and lack of strategic alloying elements makes these alloys particularly attractive. A variety of fabrication methods have been employed in the study of intermetallic compounds, and powder metallurgy processing is becoming increasingly important for obtaining desirable microstructures, improved properties, and near net shape manufacturing capabilities.
Most powder processing routes for intermetallics utilize rapidly solidified pre-alloyed powders or ribbons as starting materials, and consolidation is carried out by hot isostatic pressing or hot extrusion. Although quite successful, these methods involve many processing steps and considerable expense. High costs may be justified for certain applications by improvements in performance, however, many potential uses for these materials will be realized only if lower cost processing methods emerge.
An alternative powder processing method applicable to intermetallic compounds has received recent attention. This approach, known as reaction sintering, combustion systhesis, or self-propagating high-temperature synthesis, utilizes an exothermic reaction between powder constituents to synthesize compounds. Process advantages include the use of inexpensive and easily compacted elemental powders, low processing temperatures, short processing times, and considerable flexibility in terms of compositional and microstructural control. Depending upon thermodynamic properties and phase diagram features, a variety of reaction products are possible, ranging from highly porous to fully densified cast materials. Recent studies have demonstrated the success of this approach for fabricating nickel aluminides. Near full density Ni.sub.3 Al alloys were achieved by pressureless reaction sintering of elemental powder mixtures. It was shown that sintering was controlled by the transient liquid phase that formed during rapid exothermic heating.
Elemental iron-aluminum mixtures represent a particular challenge for powder processing because extensive compact swelling has been observed. FIG. 1 shows the iron-aluminum phase diagram. Swelling is predicted based upon phase diagram features, notably, there is a large solubility for aluminum in iron, low reverse solubility, and a large melting point difference suggesting imbalanced diffusion rates. Systems that exhibit a large driving force for compound formation are particularly susceptible to the formation of porosity during alloying. The amount of swelling observed in such systems depends upon a number of processing variables including composition, particle sizes, heating rate, green density, and temperature.
The intent of prior studies on Fe-Al was not to form intermetallic compounds as products, nevertheless, the observations reported emphasize the problems encountered in this system. Compacts containing up to 6% Al have been studied. In general, poor sintering and compact distortion were caused by exothermic compound formation and outward diffusion of aluminum. Other mixtures containing up to 15% Al have also been studied. Expansion during heating caused by outward diffusion of aluminum and pore formation at prior aluminum particle sites have been observed. Intermetallic compounds were detected in ring-shaped regions surrounding aluminum particles. Swelling was observed to increase with aluminum content above 2.5% Al. Minimum dimensional change was obtained using pre-alloyed aluminum additions, rapid heating, and isothermal sintering temperatures over 1050.degree. C.
An object of the present invention is to fabricate iron aluminides from elemental powders that overcome the aforementioned prior art problems.